Main reason:

1.The substrate does not completely remove the rust.

2.The defects on the surface is not deal with it (such as pinhole, bubble and water trace, etc) 

3.Without adding antirust primer or coating too thin, it can result in water and gas to corrade the base material.

Precaution:

1.Ensure the surface is throughly clean with the Aodalin antirust primer.

2.Ensure the paint thickness meet the demand and without demage.

Correction method:

Use the Aodalin paint remover to remove the old paint and the surface to get the bare metal surface and then use the P800 sandpaper to polish and spray again.